[adastra22]

If the universe’s causal mechanisms were infinitely fast, the entire history of the universe would play out instantly in zero time, and we’d skip straight to the heat death of the universe.

The fact that time even exists is implied by / a result of causal actions having some finite propagation time.

[Terr_]

> causal actions having some finite propagation time.

I think I know what you're getting at, but somehow the phrasing bothers me, as if there is meta-time or as if cause and effect have time between them... for the photon at light speed, time isn't passing, it's emitted and then zero "time" later it hits something very far away.

It's more like we somehow need to think of cause and effect chains that have orderings without time.

I wonder if future generations will ever look back and casually quip something about "well they believed X existed, that was their problem, it all makes intuitive sense if you just..."

[mikhailfranco]

Yes, the basic idea is that photons do not 'experience' time. They 'experience' creation, all points along their path, and absorption 'simultaneously'.

However, you have to be careful with terminology. There is no inertial frame co-moving with the photon. All we can say is: as a massive particle gets faster relative to an observer's frame, the time it experiences relative to the observer's frame becomes shorter, and in the limit, as it approaches the speed of light (but never reaches c), the experienced relative time approaches zero (but never reaches 0).

This is well explained by Don Lincoln on the Fermilab YT channel:

Do photons experience time?

https://www.youtube.com/watch?v=6Zspu7ziA8Y

[linuxdude314]

This is not true. You can easily measure TOF of a photon using an APD in Geiger mode or other detectors easily purchasable in 2024.

So what exactly are you talking about?

[lijok]

> for the photon

he’s talking about the photons frame of reference

[linuxdude314]

It’s still not true.

[BoiledCabbage]

That's really profound - thanks for sharing that insight. I've never looked at it that way before.

[jiggawatts]

That doesn’t work that way.

Imagine a universe simulated in a computer with “ticks” where the entire state is updated.

It would be different to ours, but it would work just fine.

[BoiledCabbage]

While what you say is true, it seems to me like what you're describing isn't quite the same definition of causality being infinitely fast.

The fact that causal effects happen in the next tick means some minimal time has passed. So in your definition causality can cover the entirety of a finite universe in an incredibly short amount of time (one tick). But it seems like that's not the same as covering the entirety of a finite universe in zero time. In that case, every result would happen within the same tick as its cause.

[jiggawatts]

There's more than one way to "implement a universe", but one hypothetical way to run a simulation is to alternate the simulation of forces and effects/causes.

A simulation could use particles with attributes like position, velocity, and also a "sum of forces". Then each update has the following steps:

1. Reset the sum of forces for all particles to zero.

2. For all particles, add the contribution to the forces on it from all other particles.

3. For all particles, update the next particle position based on the collected forces.

In the above, there's no intermediate state between updates, everything moves to the next position synchronously from the perspective of in-universe observers. (The external simulation can update particles one-at-a-time, but this is not an "observable" inside the simulation.)

[adastra22]

The cellular automata view of reality is entirely compatible with what I said.